JP4058823B2 - Image processing device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to processing of digital image data, and more particularly to edge processing of character images.
[0002]
[Prior art]
The image processing apparatus processes digital image data obtained by reading a document and outputs digital image data for printing. The image is reproduced on a recording medium based on the digital image data.
The image processing apparatus performs various processes on the digital image data obtained by reading the image of the document in order to better reproduce the image of the document. For character manuscripts, it is desirable to emphasize the edge of the characters in order to reproduce the character image. For this reason, various edge determination methods and data enhancement methods based on edge determination results have been proposed.
[0003]
[Problems to be solved by the invention]
When an image is formed by dividing a pixel into a plurality of sub-pixels in the main scanning direction, the resolution of the image is increased and a fine expression can be achieved. Here, at the character edge portion, the gradation of the pixel of interest is smoothed in consideration of the gradation of surrounding pixels. As a result, a multi-value gradation image can be formed even in the case of a binary image. Furthermore, the density centroid can be changed within the pixel by changing the density for each sub-pixel. However, when the smoothing process is used, there is a problem that the line width of a thin line becomes thin when the smoothing process is performed at the character edge portion to generate a multi-value gradation image.
[0004]
An object of the present invention is to provide an image processing apparatus capable of suppressing a thin line width at a character edge portion.
[0005]
[Means for Solving the Problems]
An image processing apparatus according to the present invention outputs, based on multi-valued image data, digital image data for dividing a pixel into a plurality of sub areas in the main scanning direction and forming an image in units of sub areas. An edge determination unit for identifying the edge direction of the target pixel from the difference between the multi-valued image data of the target pixel and its surrounding pixels, a line width determination unit for determining the line width of the line including the target pixel, and smoothing A symmetric smoothing filter and an asymmetric smoothing filter for processing, and multi-valued image data of the target pixel and its surrounding pixels are determined to be thin lines by the line width determining means. If this happens, select a symmetric smoothing filter and perform smoothing. Otherwise, select an asymmetric smoothing filter and perform smoothing to generate the density level of the pixel of interest. Concentration Based on the density level generated by the bell generation means and the density level generation means, digital image data for forming an image is converted into sub-pixels in the target pixel according to the edge direction of the target pixel determined by the edge determination means. Image data setting means for setting each area. As described above, the edge direction is determined by the gradation difference between the target pixel and the surrounding pixels, and is determined by the combination of the gradation differences, and the line width of the region including the target pixel is determined from the determination result of the edge direction. Further, in the density level generation means, the gradation of the target pixel is recalculated from the gradations of the peripheral pixels. Thus, even in the case of a binary image, the target pixel has a multi-value gradation, and the density centroid can be changed by controlling the density in units of sub-pixels. At this time, thinning of the thin line can be suppressed by switching the smoothing filter used by the density level generation unit according to the line width and using a symmetric filter.
The image data setting means preferably includes image data control means for setting a digital image data setting parameter for each sub-region in the target pixel in accordance with the edge direction of the target pixel determined by the edge determination means. And density level setting means for setting digital image data for each sub-region in the target pixel based on the density level of the target pixel using the digital image data setting parameters set by the image data control means.
The line width determination unit determines the line width of the line including the target pixel from the edge direction of the target pixel determined by the edge determination unit and the edge direction of the pixels around the target pixel.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an image processing apparatus according to an embodiment of the present invention will be described with reference to the accompanying drawings. In the drawings, the same reference symbols denote the same or equivalent ones.
FIG. 1 shows an embodiment of an image processing apparatus of the present invention. This image processing apparatus sub-pixels multi-value digital image data for forming an image on a photoconductor by exposing the photoconductor based on binary digital image data input from a document reading device, a computer, or the like. Generate in units and output to printer. Here, for the processing of the character edge part, the gradation difference between the target pixel and the surrounding pixels is obtained, the edge direction in the main scanning direction is identified by the combination of the gradation differences, and the obtained edge direction From the result, the line width of the region including the target pixel is obtained. Then, the edge smoothing process is switched according to the line width to suppress the thinning of the line width of the thin edge portion at the time of edge smoothing.
[0007]
More specifically, the image data output device 10 outputs binary image digital image data input from a document reading device, a computer, etc., not shown here, as 8-bit gradation data. The edge determination unit 12 uses the multi-value gradation data output from the image data output device 10 to obtain a gradation difference between the pixel of interest and its surrounding pixels, and in the main scanning direction by combining the gradation differences. Identify the edge direction at. Then, based on the determination result of the edge determination unit 12, the density level control unit 14 generates a parameter signal for controlling the density centroid in the pixel in units of sub-pixels obtained by dividing the target pixel in the main scanning direction. . On the other hand, the line width determination unit 16 determines the line width using the edge direction identified by the edge determination unit 12. The density generation unit 18 receives the multi-value gradation data of the target pixel and the surrounding pixels, and performs the smoothing process according to the line width determined by the line width determination unit 16 to change the gradation (density) of the target pixel. It is newly generated from the gradation of the peripheral pixels. The gamma correction unit 20 performs non-linear conversion of the tone data output from the density generation unit 18 and corrects the non-linear tone distortion of the print unit 26. The density level setting unit 22 controls the density level of the data corrected by the gamma correction unit 20 using the density control parameter signal generated by the density level control unit 14 to change the density centroid in the pixel. In this way, the density is controlled in units of subpixels by the density level control unit 14 and the density level setting unit 22, and the density data corrected by the gamma correction unit 20 is further converted into digital gradation data for printing in units of subpixels. Is done. The D / A converter 24 converts the digital gradation data obtained by the density level setting unit 22 into an analog signal and outputs the analog signal to the laser driving circuit of the printing unit 26. The print unit 26 modulates the intensity of the laser beam in units of subpixels based on the input signal, and forms a halftone image on the recording medium by raster scanning.
[0008]
The edge determination unit 12 determines the edge direction in the main scanning direction of the target pixel by dividing into the following four cases. Based on this determination result, it is determined in which direction the edge is moved. Here, the main scanning direction is the left-right direction. “Right edge” refers to an edge on the right side of a character, that is, an edge when a character portion is on the left side of a pixel of interest. “Left edge” refers to an edge on the left side of a character, that is, an edge when a character portion is on the right side of a pixel of interest. The “thin line edge” refers to an edge when a character portion is in the center of the target pixel, that is, when a right edge and a left edge are present in one target pixel. If none of the above applies, it is a “non-edge portion”.
[0009]
FIG. 2 is a block diagram of the edge determination unit 12. The edge is determined using, for example, a 3 × 3 pixel matrix. First, the gradation difference between the target pixel and the surrounding eight pixels is calculated and divided into a pixel having a higher density and a lower density than the target pixel. As shown in FIG. 3, in the 3 × 3 pixel matrix, V33 represents the gradation data of the target pixel, and V22, V23, V24, V32, V34, V42, V43, and V44 are eight adjacent to the target pixel. Represents pixel gradation data. In the edge determination unit 12 shown in FIG. 2, the eight gradation difference signal generation circuits 120 include gradation data V33 of the target pixel and gradation data V22, V23, V24, V32, V34, V42 of the surrounding eight pixels. , V43, V44 are inputted, and the difference (gradation difference signal) of the gradation data between the peripheral pixel and the target pixel is obtained. The combination determination circuit 122 receives the gradation difference between the target pixel and the surrounding pixels, and determines the edge direction based on the combination of the gradation differences. That is, the gradation difference between the target pixel and the surrounding eight pixels is calculated and divided into a pixel having a higher density and a lower density than the target pixel. Then, the edge direction is identified from the relationship between the density values of the target pixel and the surrounding pixels. Specifically, the combination determination circuit 122 determines the edge direction (right edge, left edge, etc.) in the main scanning direction by combining these eight gradation difference signals, and outputs the right-justified signal MARKR and the left-justified signal MARKL. Generate. The right justified signal MARKR indicates that a right edge exists, and the left justified signal MARKL indicates that a left edge exists. A logic circuit 124 including a NAND gate, two AND gates, and three selectors (selecting A when S = L) determines the edge direction from the right-justified signal MARKR and the left-justified signal MARKL, and determines the edge direction. The signal EDG is output. That is, if both MARKR and MARKL are output, EDG = “01” (thin line edge) is output, and if MARKR or MARKL is output, EDG = “03” (right edge) or EDG = “02”. If “(left edge)” is output and neither MARKR nor MARKL is output, EDG = “00” (non-edge portion) is output.
[0010]
FIG. 4 is a block diagram of the density level control unit 14. The edge direction signal EDG, which is the output of the edge determination unit 12, is input as an address signal, and eight density control parameter signals A1, A2, A3, A4, B1, B2, B3 from the table stored in the eight parameter RAMs 140 are input. , B4. The obtained density control parameter is sent to the density level setting unit 22.
[0011]
FIG. 5 shows the line width determination unit 16. Here, from the right justified signal MARKR and the left justified signal MARKL input from the edge discriminating unit 12, their continuity is judged in an area composed of a plurality of pixels including the target pixel, and the line width is set to 1 dot line (W1DOT), A line (W2DOT) is classified into a line of 3 dots or more (W3DOT). In the line width determination unit 16, first, the two shift registers 160 and 162 sequentially input the right-justified signal MARKR and the left-justified signal MARKL in the main scanning direction, respectively, and obtain MARKR and MARKL for the pixel of interest and the pixels on both sides thereof. , An AND gate, an OR gate, and the like. The logic circuit 164 determines that the line is a thin line when both MARKL and MARKR are input for the target pixel, and outputs a W1DOT signal. In addition, when MARKL and MARKR are continuously input for the pixel before the target pixel and the target pixel, or the target pixel and the next pixel, not a thin line, it is determined that the line width is 2 dots. And outputs a W2DOT signal. In other cases, when MARKKL or MARKKR is input, it is determined that the line width is 3 dots or more, and a W3DOT signal is output.
[0012]
FIG. 6 shows the concentration generator 18. The gradation data V3 of the line including the target pixel and the gradation data V1, V2, V4, and V5 of the two lines before and after the line are input to the three smoothing circuits 180, 182, and 184. The smoothing circuit 180 is a smoothing circuit for one dot line. The smoothing circuit 182 is a smoothing circuit for 2 dot lines, and includes a portion for switching between an asymmetric filter and a symmetric filter. The smoothing circuit 184 is a smoothing circuit for lines of 3 dots or more. The selector 186 outputs the output signal of the smoothing circuit 180 as the signal VF when the W1DOT signal is output, and the output signal of the selector 188 in other cases. The selector 188 outputs the output signal of the smoothing circuit 182 as the density level when the W2DOT signal is output, and outputs the output signal of the selector 1810 as the density level in other cases. The selector 188 outputs the output signal of the smoothing circuit 184 as the density level when the W3DOT signal is output, and outputs the signal V3 of the line including the target pixel in other cases. (The selectors 186, 188, and 1810 output the A input when the selection signal S is at a low level.)
[0013]
FIG. 7 is a block diagram of the density level setting unit 22. In the four density level calculation units 180, four types of combinations A1 and B1, A2 and B2, A3 and B3, A4 and B4 of the density control parameter signal are applied to the gradation data VG nonlinearly converted by the gamma correction unit 12. Are used to perform a primary operation (VH = A * (VG−B)) as shown in each block. As a result, four gradation signals VH1, VH2, VH3, and VH4 are obtained from VG. Next, the selector 182 uses the gray level signal VH1, VH2, VH3 obtained by the primary calculation in the density level calculation unit 180 using the pixel clock CLK and the double speed clock XCLK having a frequency twice that of the pixel clock. , VH4 are switched for each sub-pixel within one pixel to generate a density level signal VD. Thereby, one pixel is divided into four sub-pixels, and the density level signal VD is output for each sub-pixel.
[0014]
8 to 11 show how the density in one pixel changes for each of the edge types determined by the edge determination unit 12. As the gradation (density level) of the gradation data obtained by the non-linear conversion in the gamma correction unit 16 increases, the digital gradation data given to the four sub-pixels in one pixel changes accordingly. Indicate. In the figure, the height of the black portion in each sub-pixel represents the density level.
FIG. 8 shows the change in the case of the right edge. Here, the density control parameter signal is as follows. A1 = A2 = A3 = A4 = 4. B1 = 0. B2 = 64. B3 = 128. B4 = 192. In the figure, the gradation levels are 0, 32, 64, 96, 128, 160, 192, 224, and 255. As is apparent in the figure, since it is the right edge, the density is increased in order from the left sub-pixel. In this way, the center of gravity of the pixel density shifts sequentially from the left to the center.
[0015]
FIG. 9 shows the change in the case of the left edge. This is symmetrical to the right edge in FIG. Here, the density control parameter signal is as follows. A1 = A2 = A3 = A4 = 4. B1 = 192. B2 = 128. B3 = 64. B4 = 0. The figure shows the case where the gradation levels are 0, 32, 64, 96, 128, 160, 192, 224, and 255. As is clear in the figure, since it is the left edge, the density is increased in order from the right sub-pixel. In this way, the center of gravity of the pixel density shifts sequentially from the right to the center.
[0016]
FIG. 10 shows the change in the non-edge case. Here, the density control parameter signal is as follows. A1 = A2 = A3 = A4 = 1. B1 = B2 = B3 = B4 = 0. The figure shows the case where the gradation levels are 0, 32, 64, 96, 128, 160, 192, 224, and 255. As can be seen in the figure, since there is no edge, all four sub-pixels have the same density, and therefore the center of gravity of the pixel density is always in the center. The density increases corresponding to the gradation level.
[0017]
FIG. 11 shows the change in the case of a thin line edge. Here, the density control parameter signal is as follows. A1 = A2 = A3 = A4 = 2. B1 = 128, B2 = B3 = 0. B4 = 128. The figure shows the case where the gradation levels are 0, 32, 64, 96, 128, 160, 192, 224, and 255. Since the right edge and the left edge are thin line edges that exist at the same time, as clearly shown in the figure, first, the density of the central two sub-pixels increases corresponding to the gradation level. The center of gravity of the pixel density is always in the center. Next, the density of the two sub-pixels on both sides increases corresponding to the gradation level. The center of gravity of the pixel density is always in the center, but the density distribution gradually spreads to the left and right when the gradation level 128 is exceeded. Although FIGS. 8 to 11 mainly describe changes in the density centroid, as shown in FIG. 11, the density does not always increase from the end within the pixel of interest. Also, it is not only the density centroid that changes, but also the density distribution.
[0018]
FIG. 12 schematically shows changes in the output image when the density generation unit 18 switches the smoothing filter in the sub-scanning direction according to the line width. Consider the original image shown on the left. Usually, an asymmetric smoothing filter is used for the edge reproduction for the character edge portion (among the illustrated filters, the right filter is used when there is an image above, and the left filter is below. Used when there is an image). When an asymmetric smoothing filter is used, in the case of a line with a small line width, the line width in an image after processing by smoothing and density centroid control in sub-pixel units becomes extremely narrow compared to before processing. . In this example, a line with a width of 2 dots has a line width of about 1.4 dots.
On the other hand, if a symmetric filter is used as a filter of the smoothing circuit in the density generation unit 18 for a line with a width of 2 dots, thinning due to the smoothing process of a line with a small line width can be suppressed. In this example, a line with a width of 2 dots has a line width of at least about 1.7 dots, and is thinner and thinner than the line width of about 1.4 dots in the above comparative example.
[0019]
【The invention's effect】
In the image processing apparatus, thinning of the line width in the smoothing process of the character edge portion can be prevented.
[Brief description of the drawings]
FIG. 1 is a block diagram of an embodiment of an image processing apparatus.
FIG. 2 is a block diagram of an edge determination unit.
FIG. 3 is a diagram showing a signal distribution of nine pixels for edge determination.
FIG. 4 is a block diagram of a density level control unit.
FIG. 5 is a block diagram of a line width determination unit.
FIG. 6 is a block diagram of a density generation unit.
FIG. 7 is a block diagram of a density level setting unit.
FIG. 8 is a diagram showing a change in density with respect to a gradation level in the case of a right edge.
FIG. 9 is a diagram showing a change in density with respect to a gradation level in the case of a left edge.
FIG. 10 is a diagram illustrating a change in density with respect to a gradation level in the case of a non-edge portion.
FIG. 11 is a diagram showing a change in density with respect to a gradation level in the case of a thin line edge.
FIG. 12 is a view showing actual image data after smoothing processing;
[Explanation of symbols]
10 image data output device, 12 gamma correction unit, 14 edge determination unit, 16 line width determination unit, 18 density generation unit, 22 density level setting unit.

Claims (3)

An image processing apparatus that outputs digital image data for forming an image in units of sub-regions by dividing a pixel into a plurality of sub-regions in the main scanning direction based on multi-valued image data.
Edge determination means for identifying the edge direction of the target pixel from the difference between the multi-valued image data of the target pixel and its surrounding pixels;
Line width determining means for determining the line width of the line including the pixel of interest;
A symmetric smoothing filter and an asymmetric smoothing filter for performing smoothing processing ;
For the multi-valued image data of the target pixel and its surrounding pixels, if the line width determination means determines that the line has a narrow line width, a symmetric type smoothing filter is selected to perform the smoothing process, If not, density level generation means for selecting the asymmetric smoothing filter and performing the smoothing process to generate the density level of the pixel of interest;
Based on the density level generated by the density level generation unit, digital image data for forming an image is set for each sub-region in the target pixel according to the edge direction of the target pixel determined by the edge determination unit. An image processing apparatus comprising: image data setting means. The image data setting means includes
Image data control means for setting digital image data setting parameters for each sub-region in the target pixel according to the edge direction of the target pixel determined by the edge determination means;
A density level setting means for setting digital image data for each sub-region in the target pixel based on the density level of the target pixel using the digital image data setting parameter set by the image data control means. The image processing apparatus according to claim 1, wherein the image processing apparatus is characterized.   The line width determination unit is configured to determine a line width of a line including the target pixel from the edge direction of the target pixel determined by the edge determination unit and the edge direction of pixels around the target pixel. The image processing apparatus according to claim 1.

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